Protective wafer including inclined optical windows and device

ABSTRACT

A method for manufacturing a protective wafer including a frame wafer and an optical window, and to a method for manufacturing a micromechanical device including such a protective wafer having an inclined optical window. Also described are a protective wafer including a frame wafer and an optical window, and a micromechanical device including a MEMS wafer and such a protective wafer, which delimit a cavity, the protective wafer including an inclined optical window.

CROSS REFERENCE

The present application is a divisional application of U.S. patentapplication Ser. No. 15/790,244, filed Oct. 23, 2017, and claims thebenefit under 35 U.S.C. § 119 of German Patent Application No. DE102016221038.6, filed on Oct. 26, 2016, both of which are expresslyincorporated herein by reference in their entirety.

BACKGROUND INFORMATION

The present invention relates to a method for manufacturing a protectivewafer including a frame wafer and an optical window, and to a method formanufacturing a micromechanical device including such a protective waferhaving an inclined optical window.

MEMS components must be protected against harmful outside environmentalconditions, for example the penetration of particles and the like. Aprotection against mechanical contact or destruction and for enablingthe separation from a wafer assembly into individual chips by sawing isalso necessary. In many cases, the setting of a certain atmosphere (gastype and gas pressure; vacuum also) must be made possible with the aidof a hermetic encapsulation.

The encapsulation of MEMS components with the aid of a cap wafer, whichhas cavities and through-holes, in the wafer assembly is an establishedprocess. For this purpose, a cap wafer is aligned with the waferincluding the MEMS structures and joined thereto. The joining may takeplace both with the aid of anodic bonding and direct bonding (joiningagent-free joint between glass and silicon), via eutectic joining layersand via glass solders or adhesives. The MEMS component is usuallysituated under the cavities of the cap wafer, and the electrical bondpads for connecting the component to thin wires are accessible via thethrough-hole in the cap wafer.

For optical MEMS (MOEMS), such as for micromirrors, the above-describedprotection and additionally a transparent window having a high opticalquality and, if necessary, also including special optical coatings, arenecessary. In isolated instances, through-holes for the electricalconnection are also implemented in the caps.

When optical beams pass through the transparent window, reflections arecreated at the interfaces. When the stationary reflections are in thescan range of the μmirror, their intensity exceeds that of the projectedimage and thus has an interfering effect. These interfering reflectionsmay only be reduced in their intensity by an anti-reflection coating ofthe optical window. Since the mirror in general pivots or is deflectedsymmetrically around its rest position, the reflection is always in thescan range when the optical window is in parallel to the rest positionof the mirror surface and the distance between the mirror plane and theoptical window is small. This is frequently the case with MEMS. The onlyoption to avoid the interference by the reflections is to guide them outof the scan range in that the optical window and the mirror surface (inrest position, i.e., in the non-deflected state) are not situated inparallel to one another. Two options exist for this: inclination of theoptical window or inclination of the rest position of the mirror withrespect to a main plane of the MEMS component. Both options are alreadyknown from publications and from patent specifications in variousspecific embodiments. Inclined windows for separated chips are knownfrom EP 1688776 A1, for example.

Inclined windows or also other window shapes with which the reflectionsare avoidable are described for wafer level packaging in U.S. PatentApplication Publication No. 2007/0024549. The three-dimensional surfacestructures (e.g., inclined windows) described in the latter patentspecification are to be manufactured from a transparent material (glassor plastic material) in a wafer assembly. The methods with the aid ofwhich the three-dimensional structures may be manufactured are eithervery expensive or do not result in the necessary optical quality. Wafersincluding corresponding three-dimensional structures are moreoverproblematic during processing (e.g., during wafer bonding) since thestructures may be damaged in the process.

Other methods for manufacturing protective caps having inclined opticalwindows are described, for example, in the German Application Nos. DE102010062118 A1 and DE 102012206858 A1. However, these methods are alsostill relatively expensive for high volume applications, or the opticalwindows have only a limited optical quality.

SUMMARY

The present invention relates to a method for manufacturing a protectivewafer including a frame wafer and an optical window, including thesteps:

A) providing a frame wafer having a front side and a rear side;

B) creating a through-opening from the front side to the rear side withthe aid of etching;

C) etching first trenches into the frame wafer from the front side;

D) closing the through-opening with an optical window; and

E) etching second trenches into the frame wafer from the rear side, insuch a way that the first trenches form an interdigital structure withthe second trenches, whereby a meander-shaped spring structure is formedin the frame wafer, which surrounds the through-opening, the springstructure including a first spring area having a first spring length anda second spring area having a second spring length, the first springlength being greater than the second spring length.

Advantageously, the method creates a protective wafer including anoptical window which is flexibly suspended and whose position is thusadjustable.

One advantageous embodiment of the method according to the presentinvention provides that, prior to step D, a first recess is created onthe front side or also for a second recess to be created on the rearside of the frame wafer, in particular with the aid of KOH etching, andthe through-opening is situated in the area of the first recess or alsoof the second recess. In this way, the through-opening mayadvantageously be created quickly and cost-effectively. Advantageously,in this way the optical window may also be situated recessed on theframe wafer, whereby the overall depth is kept low, and the opticalwindow is protected against mechanical damage such as scratches.

One advantageous embodiment of the method according to the presentinvention provides that, in step D, the frame wafer in an edge areaaround the through-opening is peripherally provided with a glass solder,and the optical window is situated on the frame wafer with the aid ofthe glass solder.

One advantageous embodiment of the method according to the presentinvention provides that, in step D, the optical window is arranged onthe frame wafer with the aid of flip chip technology.

The present invention also relates to a method for manufacturing amicromechanical device including a protective wafer manufacturedaccording to the present invention having an inclined optical window,including the following steps:

F) joining the frame wafer to a MEMS wafer, a hermetically sealed cavityhaving an internal atmospheric pressure being formed, which is delimitedat least by the spring structure and the optical window; and

G) subjecting the micromechanical device to an external atmosphericpressure, which is different from the internal atmospheric pressure ofthe cavity, whereby the optical window is deflected out of a restposition in parallel to the MEMS wafer in such a way that the opticalwindow is positioned at an incline with respect to the MEMS wafer inthat the optical window is farther deflected relative to the MEMS waferin the vicinity of the first spring area than in the vicinity of thesecond spring area.

The manufacturing method offers an option to create micromechanicaldevices including inclined optical windows to avoid interferingreflections.

The present invention also relates to a protective wafer including aframe wafer and an optical window, the optical window being situated ina through-opening of the frame wafer, the frame wafer including firsttrenches on a front side and second trenches on a rear side, which aresituated as an interdigital structure and form a meander-shaped springstructure, which surrounds the through-opening, the spring structureincluding a first spring area having a first spring length and a secondspring area having a second spring length, the first spring length beinggreater than the second spring length.

The present invention also relates to a micromechanical device includinga MEMS wafer and such a protective wafer, which delimit a cavity, theprotective wafer including an inclined optical window, the cavity havingan internal atmospheric pressure which is different from the externalatmospheric pressure of the cavity, the optical window being deflectedout of a rest position in parallel to the MEMS wafer in such a way thatthe optical window is positioned at an incline with respect to the MEMSwafer in that the optical window is farther deflected in the vicinity ofthe first spring area than in the vicinity of the second spring area.

Advantageously, the present invention creates a cost-effectivemanufacturing method suitable for high volume series-production. Theinclined optical windows may be manufactured with the aid of processescustomary in MEMS and semiconductor technology. In particular with arecessed arrangement of the optical window on the frame wafer, it ispossible to avoid scratches, particles and damage on the optical windowduring the further manufacture and during the later operation of thedevice.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-f show a method according to the present invention formanufacturing a micromechanical device including an inclined opticalwindow based on cross-sectional views of a chip.

FIG. 2 shows a top view onto a detail of a micromechanical deviceaccording to the present invention in the form of a chip including anoptical window and circumferential resilient elements havingcorresponding trenches.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The manufacturing method is described hereafter for a protective waferincluding cavities, through-holes and a transparent inclined window inoptical quality. The sequence of the manufacturing method is describedhere only by way of example and may also take place deviating therefrom.The method is described by way of example based on FIGS. 1a through 1fin a sectional view and based on FIG. 2 in a top view.

The micromechanical device according to the present invention includes aprotective wafer 10 and a MEMS wafer 400. Protective wafer 10 is made upof a frame wafer 100 and optical windows 300. Frame wafer 100 ispreferably made of silicon, and the optical windows 300 are made ofglass having a thermal coefficient of expansion adapted to silicon.

In a first step, depressions are introduced on front side 110 and on arear side 120 of frame wafer 100, in particular with the aid of KOHetching (FIG. 1a ). This is first recess 115 on front side 110, andsecond recess 125 on rear side 120. Thereafter, a through-opening 150 isintroduced (FIG. 1b ), over which optical window 300 is to bepositioned. Through-opening 150 may be achieved by trench etching oralso with the aid of KOH pre-etching of this area, prior to thetwo-sided KOH etching of the depression. First recess 115 on front side110 is used to situate optical window 300 recessed with respect to thewafer surface of frame wafer 100. In this way, it is ensured that it isnot possible to damage optical window 300 after the insertion. Scratchesand the like due to mechanical action are thus avoided. In the sametrench etching process step, first trenches 211 are circumferentiallyintroduced around the area of optical window 300. On rear side 120, anetching mask 30 for rear-side second trenches 221 is applied (FIG. 1c ).Second trenches 221 extend between first trenches 211 of front side 110and form an interdigital structure with these. The trench depth on frontside 110 and rear side 120 should be selected in such a way thatlaterally and vertically only a thin remaining thickness of the siliconremains. The front side and rear side trenches, in the cross section,result in a spring structure 200, a meander-shaped element which due tothe thin silicon walls acts like a bellows (accordion). Bellows 200 maybe both expanded in the wafer plane and be bent out of or into the waferplane. The stiffness regarding the expansion and bending may be set viathe number of the trenches and via the wall thickness of the silicon.The design of bellows 200 is to be implemented accordingly.

The insertion of optical windows 300, which previously werecircumferentially provided with glass solder 310, takes place with theaid of flip chip technology even before the introduction of secondtrenches 221 (FIG. 1c ).

After the insertion of optical windows 300, frame wafer 100 completelyequipped with windows is heated on a heating plate, and when thesoftening temperature of glass solder 310 has been reached, window 300is pressed onto the window frame of frame wafer 100 with the aid of apressure difference between the front side and the rear side. Glasssolder 310, which is present between glass window 300 and frame wafer100, softens and thereby spreads. After cooling, a hermetic joint isthus established between glass window 300 and frame wafer 100. Insertingoptical windows 300 even before the rear-side second trenches 221 arecompleted is advantageous to avoid or minimize a deflection of thebellows out of the wafer plane due to the applied pressure difference.

This is followed by the etching of second trenches 221 with the aid ofthe already completed mask 30 (FIG. 1d ). Advantageously, the mask ismade of oxide and thus does not have to be removed after the trenchetching. As a result, protective wafer 10 is created.

For the creation of a hermetic bond of protective wafer 10 with a MEMSwafer 400, in particular an actuator or sensor wafer, a glass solder310, for example, is applied to rear side 120 of frame wafer 100 (FIG.1d ).

The creation of the wafer assembly is best carried out usingMEMS-customary wafer assembly processes and equipment. An underpressure(up to a vacuum) or an overpressure with respect to the outside worldmay be set in cavities 500 between frame wafer 100 and MEMS wafer 400.This pressure difference achieves a resulting force on optical window300 and thus a deflection of the resilient elements of spring structure200. An underpressure (vacuum) in cavity 500 results in a deflectioninto the wafer plane (FIG. 1e ), and an overpressure results in adeflection beyond the wafer plane (FIG. 1f ).

To achieve a tilt of optical window 300, spring structure 200 is to bedesigned in such a way that a first spring area 220 is provided, forexample on three sides of optical window 300, which is ensured a largefirst spring length 225 a low stiffness with respect to a movement outof the wafer plane. On the fourth window side, spring structure 200 isto be designed in such a way that such a movement preferably does nottake place. Correspondingly, a second spring area 240 having a smallsecond spring length 245 is situated here. On this side, however,bending should be possible in second spring area 240, which allows aninclined position of optical window 300. The tilt angle may essentiallybe set via the pressure difference, i.e., the difference between theinternal atmospheric pressure in cavity 500 and the external atmosphericpressure, the stiffnesses and spring lengths of the bellows elements.FIG. 2 schematically shows such a design in a top view.

The tilting or inclined position of optical windows 300 is only providedwhen the pressure difference between cavity 500 and the outside world ispreserved. The degree of the deflection of optical windows 300 may thusbe used as a test criterion for the tightness of cavity 500.

In this way, the tightness test may be carried out both during themanufacturing processes and later during operation of the device. In theoperating mode, the interfering, stationary reflection appears in thescan range of the micromirror if a component is not tight, and thus ifan optical window is not inclined.

LIST OF REFERENCE NUMERALS

10 protective wafer

100 frame wafer

110 front side

115 first recess

120 rear side

125 second recess

150 through-opening

300 optical window

310 glass solder

30 rear-side etching mask

200 spring structure

211 first trenches

221 second trenches

220 first spring area

225 first spring length

240 second spring area

245 second spring length

400 MEMS wafer

500 cavity

What is claimed is:
 1. A protective wafer, comprising: a frame wafer;and an optical window situated in a through-opening of the frame wafer;wherein: the frame wafer includes at least one first trench on a frontside of the frame wafer and at least one second trench on a rear side ofthe frame wafer; the at least one first trench and the at least secondtrench are situated as an interdigital structure and form ameander-shaped spring structure that surrounds the through-opening; afirst spring area of the spring structure, that is at a firstcircumferential position relative to the through-opening, has a firstspring length in a respective direction from the first spring areatowards the through-opening; a second spring area of the springstructure, that is at a second circumferential position relative to thethrough-opening, has a second spring length in a respective directionfrom the second spring area towards the through-opening; and the firstspring length is greater than the second spring length.
 2. Amicromechanical device, comprising: a MEMS wafer; and a protectivewafer, wherein: the protective wafer includes a frame wafer and anoptical window situated in a through-opening of the frame wafer, theframe wafer includes at least one first trench on a front side of theframe wafer and at least one second trench on a rear side of the framewafer; the at least one first trench and the at least second trench aresituated as an interdigital structure and form a meander-shaped springstructure that surrounds the through-opening; a first spring area of thespring structure has a first spring length; and a second spring area ofthe spring structure has a second spring length; the first spring lengthis greater than the second spring length; the MEMS wafer and theprotective wafer delimit a cavity; the cavity has an internalatmospheric pressure that is different from an external atmosphericpressure; and the optical window is arranged to be deflected out of arest position at which the optical window is parallel to the MEMS waferinto a position at which the optical window is at an incline withrespect to the MEMS wafer in that the optical window is fartherdeflected in a vicinity of the first spring area than in a vicinity ofthe second spring area.
 3. The micromechanical device of claim 2,wherein the protective wafer further includes a glass solder connectingthe optical window to the frame wafer.
 4. The protective wafer of claim1, wherein, due to the first spring length being greater than the secondspring length, the spring area is deflectable to a greater degree thanthe second spring area.
 5. The protective wafer of claim 1, wherein, dueto the first spring length being greater than the second spring length,an edge region of the optical window at which the optical window isattached to the first spring area is deflectable to a greater degreethan another edge region of the optical window at which the opticalwindow is attached to the second spring area.
 6. The protective wafer ofclaim 1, further comprising a glass solder connecting the optical windowto the frame wafer.